The pneumatic tire is unsurpassed in providing load support with maximum shock absorption, not only for automobiles, trucks, and aircraft but also for lift trucks, dock vehicles, military vehicles, municipal service equipment, and the like. This superior performance results from a combination of the properties of the reinforced rubber casing and gas at a proper pressure. Of course, a major drawback to the use of gas-filled pneumatic tires is the inconvenience and danger posed by puncturing of the tire. Tire failure, as in blow-outs, can result in human injury and equipment damage. A slow gas leak results in improper inflation leading to premature tire wear and increased rolling resistance. Particularly in an industrial environment where scrap material may be strewn along the floor or roadway, the use of gas-filled pneumatic tires must either be avoided or the contaminated area must be constantly swept clean.
A variety of solutions designed to prevent or mitigate the puncture of pneumatic tires have been proposed and used. Liners of various types have been provided in the tire or between an inner tube and the tire casing serving to mitigate the effects of penetration of the tire. Some heavy industrial pneumatic tires have been fitted with an impenetrable metal chain barrier. A more prevelant method for overcoming the problem is converting pneumatic tires to solid or semi-solid composite tires. Such tires have gained wide acceptance for certain mining, industrial and construction uses where the added weight, and somewhat inferior dynamic performance, could be tolerated for permanent protection from flat tires.
Until recently, such solid, deflation-proof tires have depended on the presence of a foamed elastomer filling. For example, Lambe U.S. Pat. No. 3,022,810 describes a pneumatic tire filled entirely with an intrinsically compressed resilient foam which is produced in situ, that is, directly within the tire, exemplified by a polyurethane foam in which carbon dioxide bubbles are evolved during reaction producing a rubber-like polymer foam within the casing. Altorfer U.S. Pat. No. 3,112,785 adds a conventional foaming agent to liquid polyurethane foaming material as it is pumped into a tire casing, with resultant in situ formation of a foamed polyurethane. Altorfer then injects a liquid antifreeze solution into the tire to be dispersed along with air into the cellular structure. Talcott et al U.S. Pat. No. 3,381,735 describes a deflation-proof vehicle tire in which synthetic rubber filler material is foamed in place and then vulcanized. Lombardi et al. U.S. Pat. No. 3,605,848 describes a tire having its casing filled with a microcellular, open cell urethane core. Water is described as serving as a blowing agent in the production of carbon dioxide upon reaction with the isocyanate component of the polyurethane precursor liquids. Again, when the precursor polymer material is injected into the tire casing, it results in the formation of a polymer foamed in situ.
Because the foam fillings in such tires are easily flexed, the tires have serious disadvantages related to excessive heat build-up within the tire and filler break-down during service, decreasing casing support with the possibility of severe casing damage. Moreover, by generating gases in situ, volumes and pressures are undefinable and unpredictable. In order to obtain sufficient uniformity to achieve predetermined volume and pressure levels, "factory installation" is required with expensive and inordinate controls to assure uniformity from tire to tire. Because of the drawbacks to the foregoing utilization of foamed polymers as fillers, Gomberg in U.S. Pat. No. Re. 29,890 proposed a pneumatic tire in which the casing was filled with a void free elastomeric material. Specifically, the elastomeric material was produced in the essential absence of foam producing material in the reaction zone. Because it has less deflection than foam tires, superior heat build-up characteristics were obtained. Additionally, the elastic material was found to have a Durometer hardness in the range of about 25-43 on the A Scale. Generally, a Durometer hardness of at least 20, preferably about 30 on the A Scale is satisfactory. However, because the Gomberg elastomer entirely fills the tire casing without voids, a significant drawback is the resultant very high cost of filling a tire. Efforts to reduce that cost by diluting the filler material with extender oil can result in a sharp decrease in hardness.
The present invention proceeds by adding water as a reactant to produce carbon dioxide in the reaction zone but curing the elastomer under conditions whereby the carbon dioxide is dissolved in the elastomer to produce a substantially void-free elastomeric filling material. Generally, for most tires, those that stretch, a pressure of at least 25 psi is required to prevent bubble formation in the tire.
As a result of the water reaction, a polyurea-containing polyurethane elastomer is obtained having superior hardness characteristics. Oil can be added while maintaining a Durometer hardness of at least 20 on the A scale. While a polyurea-containing elastomer generally has less oil compatability on a weight basis than an all-urethane-containing elastomer, because of its very high Durometer hardness, it actually has higher oil compatability for a particular level of Durometer hardness. In other words, whereas urethane systems can be oil-extended at a useful hardness level, the level of oil extension is limited by large decreases in hardness. On the other hand, elastomer prepared in accordance with the present invention can be extended by as much as 50 weight percent with oil and still yield a Durometer hardness of 30; at lower hardness levels, up to 60 weight percent oil can be added without bleeding from the elastomer at room temperature.
It will be appreciated that the result is a large savings in cost in filling a tire. Additionally, because water is used as a reactant, one can operate without the stringent moisture-free conditions required in producing the Gomberg elastomer. Furthermore, one can incorporate other diluents which contain moisture which cannot be incorporated in the Gomberg tire because of the moisture content. For example, one can add substantial quantities, up to 50 weight percent, of rubber buffing dust, ground off the tires before retreading, thereby yielding additional cost savings.
In addition, the fill appears to resist heat build-up. It is hypothesized that because the fill contains CO.sub.2 under pressure, as temperatures during use rise, e.g. to 200.degree. F., the CO.sub.2 content serves to increase pressure, decreasing deflection and increasing rebound, serving to limit heat build-up.
The urethane is prepared by reacting a polyol with an organic polyisocyanate and water, as described above. In a particular embodiment, the polyol is a triol having a molecular weight of at least 3,000; it has been found that higher molecular weight polymers are more compatible with higher oil levels. A particularly useful polyisocyanate is toluene diisocyanate. Another particularly useful polyisocyanate is polymethylene polyphenylisocyanate. In addition, one can incorporate a small amount, up to about 5 weight percent, of an inorganic absorbent for the carbon dioxide, such as calcium oxide or hydroxide, aluminum trihydrate, zinc oxide or the like.